A robust controller is designed by applying the H-infinity optimal control theory to the xenon control for the load-following operation of a nuclear reactor. The set of reactor model equations for controller design is a stiff system. This singularly perturbed system arises from the interaction of slow dynamics modes (iodine and xenon concentrations) and fast dynamics modes (neutron density, fuel and coolant temperatures). The singular perturbation technique is used to overcome this stiffness problem. The design specifications are incorporated by the frequency weights using the mixed-sensitivity problem approach. The robustness of H. control is demonstrated by comparing it with linear quadratic Gaussian (LQG) control in the case of a measurement delay of the power measurement system. Since the gains and phase margins of H-infinity control are larger than those of LQG control, the H. control is expected to provide excellent stability robustness and performance robustness against external disturbances and noises, model parameter variations, and modeling errors as well as hardware failures. It may also provide a practical design method because the design specifications can be easily implemented by the frequency weights.